The invention relates to a method and a device according to the respective characterizing portions of the independent claims. Accordingly, the invention relates to a solar cell arrangement.
Solar cell arrangements find a wide variety of applications. Large-surfaced solar cell arrangements are used in photo-voltaic systems, for example, which can provide sufficient energy for consumers with a higher demand. In this case, the costs of the cells often play a subordinated role because they are not significant compared to the costs required for a connection to a public supply system, or because there is no useful alternative, as in the case of aerospace applications.
Solar cell arrangements are also used in a variety of small devices, currently having a low output, such as pocket calculators and wristwatches. However, in principle, it is also feasible to use solar cell arrangements in electronic consumer goods appliances having a slightly higher energy requirement than a pocket calculator or a watch. For example, solar cell arrangements could be considered for charging and operating portable computers and cell phones independent of the network where the operating voltage usually ranges between 6 and 12 V.
Technically, this is feasible, because a cell phone battery, for example, which provides approx. 2 Watts for a call, requires a charging voltage of approx. 7 Volt and a total charge of approx. 550 mAh to 1200 mAh. In order to keep the cell phone functional merely by means of insolation a surface of approx. 50 cm2 is available on the rear.
With solar cell arrangements based on silicon technology having said surface and assuming an efficiency of only approx. 14% and small series connection losses a voltage of more than approx. 7 Volt and currents of more than 100 mA at the maximum power point are achievable so that it is possible, in principle, to achieve a charge by means of insolation of sufficient duration.
The cells to be installed, however, should not affect the size or compactness, nor should they be too expensive in order to be used in consumer products. To meet the first requirement relatively small solar cells of a few square centimeters have to be connected which requires very small modules instead of the large-surfaced solar cell modules commonly used in photo-voltaic systems. These must be mounted so as to be cost-effective.
Previously, the use of solar cell arrangements often failed because the costs were too high. This was also due to the high costs of series connection. In principle, in series connection the base of the first cell has to be connected conductive with the emitter of the second cell. Typically, the base is arranged on one solar cell surface, the emitter on the opposite surface.
When solar cells on the basis of crystalline silicon wafers are used for solar cell arrangements there are several options, in principle, according to the state of the art to achieve the series connection. For example, the cell front surface of a solar cell can be connected electrically conductive with the rear surface of the following cell via tin-coated copper strips or the like. Further, it is known in the art from xe2x80x9cDirect Conversion of Energyxe2x80x9d by K. J. Euler, Karl Thiemig KG Publishers, Munich, 1967, page 55, to arrange the individual solar cells slightly overlapping in the manner of shingles or roof tiles and then to electrically connect the base rear contact of one cell with the emitter front contact on the following cell. From the article xe2x80x9cEmitter Wrap-Through Solar Cellxe2x80x9d by J. M. Gee et al., Proc. 23rd IEEE PVSC, Louisville, 1993, pages 265-270, it is known to provide a solar cell with front and rear emitter of the n+type which encompasses a p-doped base material. While the emitter on the front has to be provided only with an anti-reflex coating two contact grids are provided on the rear, one of the p-type for contacting the base and one of the n-type for contacting the emitter, respectively. Both grids form a pattern of interlinked fingers connected at one end via so-called bus bars. In order to connect the emitter on the front with the emitter on the rear or with the emitter contact grid provided there, holes are made in the substrate by means of laser pulses, which are then doped and filled with metal using a selective method so as to produce a conductive connection. While the publication discusses the series resistance of an individual element, it does not specify how multiple cells can easily be connected.
It was further proposed in an article published in the Internet entitled xe2x80x9cThe crystalline-silicon photo-voltaic RandD Project at NREL and SNLxe2x80x9d by J. M. Gee and T. F. Ciszek at http://www.sandia.gov, to provide a module assembly concept where all cells of a module are encapsulated and electrically connected in one single step. For this purpose, the cells are contacted on the rear where a rear module surface plane includes both the electrical circuit and the encapsulation material in one single piece, and a one step method is provided for arranging said components into a module.
For this purpose, the cells contacted on the rear are placed on a plane base having a pre-formed electric connecting pattern and deposited on the rear surface beyond said base. This requires handling individual cells, which is problematic in the above mentioned applications because of the size of the cells.
The article xe2x80x9cAn industrial multi-crystalline ewt solar cell with screen printed metallizationxe2x80x9d by A. Schxc3x6necker et al., 14th European Photo-voltaic Solar Energy Conference, Barcelona, 1997, pages 796 and following, specifies connecting a number of cells in that they are contacted via bus bars, for example using the above described emitter-wrap-through technology, and subsequently connecting the bus bars of the individual modules via electrical conductors. The article does not specify how the individual modules should be handled.
The article xe2x80x9cAdvances in thin film PV technologiesxe2x80x9d by H. A. Aulich, 13th European Photo-voltaic Solar Energy Conference, Nice, France, Oct. 23 to 27, 1995, pages 1441 and following specifies a number of different thin film solar modules consisting primarily of materials other than silicon. Also, the use of amorphous silicon is described where reference is made to the so-called photo degradation, i.e. a deterioration in the effectiveness over time.
A further method of contacting is known from the article xe2x80x9cHigh efficiency (19.2%) silicon thin-film solar cells with interdigitated emitter and base front-contactsxe2x80x9d by C. Hebling et al., 14th European Photo-voltaic Solar Energy Conference, Barcelona, Spain, Jun. 30 to Jul. 4, 1997.
It proposes a so-called SOI structure, i.e. xe2x80x9csilicon on insulator structurexe2x80x9d where the actual cell is applied to an insulating layer. The described method is comparatively expensive, however.
From the book xe2x80x9cSilicon solar cellsxe2x80x94advanced principles and practicexe2x80x9d by Martin A. Green, ISBN 0 7334 09946, published by the Center for photo-voltaic devices and systems, University of New South Wales, Sydney, NSW 2052 in March 1995, solar cells with a so-called buried structure are known, where grooves are made in the material by means of a laser, mechanical cutting wheels or other mechanical or chemical means, and the grooves are chemically cleaned. Subsequently, a strong doping agent is applied and the grooves are filled with metal. Contacts are obtained thus after taking further steps. The book also mentions the above mentioned finger-like cells contacted on the rear as well as point contacted solar cells where, instead of a p-conductive bus bar, p-conductive contact points are distributed over the cell so as to better determine certain cell properties.
From the U.S. Pat. No. 4,612,408, an interconnected solar cell array is known which is produced in that a number of the arrangements are formed on the semiconductor surface, grooves are formed in the surface which, in part, extend into the substrate, an oxide layer is formed on selected sections of one surface and on the surfaces of the grooves, the grooves are filled with an insulating material, metal connections are produced between adjacent components extending over the grooves and the insulating material, an insulating support is mounted on one surface and then a separation beginning at the other surface side through the wafer into the grooves so as to separate the adjacent arrangements while the metal connections remain. Such a separation is problematic.
The U.S. Pat. No. 4,300,680 describes a series of strip-shaped semiconductor connections arranged on one of the two surfaces as a semiconductor substrate having a high ohmic resistance, where said connections alternately have p+ and n+ conductive characteristics and are parallel with respect to each other and are interspaced at intervals in such a way that a semiconductor connection having a p+ conductivity characteristic on a surface of the semiconductor substrate is always opposite a semiconductor connection having an n+ conductivity characteristic on the other surface, and printed strip conductors are arranged alternately on one and the other surface of the semiconductor substrate where said strip conductors always connect a row of solar cell connections with an adjacent row.
The U.S. Pat. No. 5,391,326 describes a photo-voltaic solar cell array which is produced monolithic without a supporting substrate in that a system of channels is formed of one side of a substrate so as to define separate cell surfaces, the channels are filled with an insulating filler material adhering to the substrate material and providing it with structural integrity, and then channels are provided from the opposite side of the substrate so as to provide an air gap isolation system between adjacent cells. Series connections between adjacent cells are provided in that the front surface of a cell is connected via the filler material with the volume semiconductor for the following cell where the connection is provided through the volume semiconductor itself to the rear electrodes of each cell. Said contacting provides only a small contacting surface resulting in high intermediate resistances. Furthermore, the filler material used is polyimide and represents a significant cost factor in the production.
The U.S. Pat. No. 5,164,019 describes an array of series-connected cells formed in a monolithic semiconductor substrate which are electrically insulated from each other in that grooves extending partially through the substrate are formed in a first surface between cells and the substrate is then broken by the bottom of the grooves to an opposite main surface. A metallization connecting the cells provides the physical integrity of the cell field after breaking the substrate. The grooves can be formed before or after the production of the cells is completed. It is problematic that the braking of the cells has to be defined and the mechanical stability is ensured only by metal contacts which are typically thin.
The U.S. Pat. No. 5,024,953 describes an opto-electrically transmitting element and a method for producing the same using a corrugated semiconductor substrate so as to produce an opto-electronically transmitting element. The element has a reduced effective thickness and an improved opto-electronic conversion efficiency while its mechanical stability remains intact. A form is provided therefor having a zigzag or a meandering shape in profile where said wavy or corrugated structures are continuous.
DE 44 263 47 specifies a flat component having a grid of through-holes where said through-holes are produced in that on both the front and rear of a disc-shaped circular body a number of preferably equidistant, parallel, particularly V-shaped channels are formed where the channels of both sides are at an angle relative to each other and have a depth so as to produce through-holes on the intersecting points of the channels. DE 44 263 47 is primarily concerned with the question of how such grids can be produced. It proposes using as a compensating material, for example, mono-crystalline silicon disc wafers cut from cast silicon blocks, multi-crystalline silicon strips produced by means of precipitation from a silicon melt on a graphite network, etc. The above document discusses contacting the respective solar cells, but does not indicate how the used voltage can easily be increased by means of interconnecting multiple solar cells.
A solar cell panel is known from the U.S. Pat. No. 3,330,700 having a base structure that includes a metal substrate, an insulating layer on top of the substrate and a coating of adhering material on top of the insulating layer where a field of four solar cell rows is arranged parallel on the coating of adhering material and the cells are arranged such that the p-layers of a row of cells are adjacent to the n-layers of the following row of cells. The cells are isolated and completely separated from each other and are connected via metal strips.
The U.S. Pat. No. 3,903,427 proposes a method for connecting and contacting so as to reduce the loss of power in the solar cell. Accordingly, a solar cell is produced from a semiconductor wafer where the light-sensitive semiconductor material has a top and a bottom surface. Holes are extending through the semiconductor wafer from the top to the bottom thereby producing an electrical contact from the top to the bottom where the contact leading down from the top is electrically insulated against the contact on the bottom so as to allow collecting the current. However, it is not specified how a particularly favorable series connection can be produced particularly advantageously.
A further solar cell arrangement is known from the U.S. Pat. No. 3,411,952 where a number of solar cells are arranged and soldered to metal strips. By appropriately selecting the length and width of said metal strips on which the individual solar cell modules are placed the desired voltage is generated in accordance with the U.S. Pat. No. 3,411,952.
The U.S. Pat. No. 4,129,458 describes a solar cell arrangement consisting of multiple interspaced longish unit cells of a field from a single wafer which is longitudinally grooved and consisting of a substrate material of a first conductivity type, having adjacent side walls of adjacent units at each groove formed between units. Transversely in such groove formations, the side walls of every second groove are formed having areas of a second conductivity type so as to result in only one connection between the first and second conductivity types at or near the surface of each unit which is exposed to the radiation.
According to a general form, the grooves extend over the full distance between the top and bottom wafer surfaces while forming discrete one-cell units. According to another general form, every second groove ends near, but before the top surface, so as to define discrete double cell units. The units are series-connected in that ohmic connections between the area of the second conductivity type of one unit and the area of the first conductivity type of an adjacent unit are produced. This is intended to allow a comparatively simple and economical production. This cell arrangement is intended to build a better solar cell field from a single piece of substrate material where the individual units of the field remain in precisely the same positional relationship as in the individual original piece. According to the U.S. Pat. No. 4,129,458, the grooves or gaps between the units can be filled with insulating material, or they remain partially or fully open, while a support is used to provide the relative alignment. With regard to forming the grooves, it is merely proposed that they should not fully extend continuously from one surface to the opposite one. Thus, connections extending over the full length of the adjacent side may remain.
The U.S. Pat. No. 4,179,318 having the same author as the above mentioned U.S. Pat. No. 4,129,458, also describes a solar cell arrangement. Again, the groove formation described therein does not differ from the one described in U.S. Pat. No. 4,129,458.
A further arrangement of a semiconductor solar cell is known from the U.S. Pat. No. 4,283,589 which, for increasing the efficiency of a solar cell, proposes to provide a substrate body with a number of laterally interspaced longish grooves having opposing slanting side walls diverging in a certain way.
The grooves in the semiconductor substrate are also provided so as to allow a certain solar cell field connection. In an exemplary embodiment shown in the U.S. Pat. No. 4,283,589, cell units are formed from a single source substrate provided with grooves via which adjacent cell units are spaced apart by a predetermined width. The grooves extend fully through the semiconductor wafer and the mechanical integrity is provided either by means of filling the grooves with a suitable material or by means of a supporting oxide layer extending below the groove.
A further solar cell is known from the U.S. Pat. No. 4,352,948 where grooves are provided. Among other things, it is proposed that the grooves provided in a single wafer have the same length and end shortly before the opposite sides. However, the individual grooves do not extend through the material over its full depth.
The U.S. Pat. No. 4,376,872 specifies a solar cell having a number of discrete voltage-generating areas which are formed from a single semiconductor wafer and are interconnected in such a way that the voltages of the individual cells will sum up. The unit cells include doped areas of opposite conductivity types separated by a gap. Said known solar cells are produced in that V-shaped grooves are formed in the wafer and the wafer is subsequently aligned such that ions of one conductivity type can be implanted on one side of the groove while the other side is screened.
A metallizing coat is applied and selectively etched away so as to provide connections between the unit cells.
A solar cell is known from the U.S. Pat. No. 5,067,985 using an anisotropic etched silicon crystal wafer oriented in a certain way. The solar cell structure includes top edges between cell channel walls so as to improve the ability to capture the light. However, the publication does not discuss the question of how solar cell units can be produced from a single waver and connected so as to obtain a higher voltage.
A further solar cell arrangement is known from the U.S. Pat. No. 5,641,362 using a certain contacting system so as to provide a cost-effective solar cell arrangement. Accordingly, two contact patterns each consisting of parallel strips which are connected to form a bus bar on one end are . . . on the rear of the solar cell [sentence incomplete]. The wafer is uniform, however, except for some possible superficial modifications.
From DE 35 29 341 A1 a solar cell module is known having multiple single cells arranged on a support body and connected electrically conductive where said support body is equal in area to the single cells, including the separation slits between them. The contacts on the rear of the solar cells are connected electrically conductive with associated connection contact surfaces on the rear of the support body via contacts in the support body. A suitable support body, for example, is a glass fiber reinforced synthetic material having a thickness of 0.3 mm. A connecting contact is provided on the support body and a large surfaced solar cell is soldered thereto which is equal in area to the whole of the support body.
Subsequently, the semiconductor disc mounted on the support body is divided by means of separating cuts so as to prevent the necessity for handling each individual cell separately. The use and the complex configuration of the support body, however, are undesirable, because they are cost-intensive.
WO 89/05521 specifies a solar cell arrangement consisting of a number of interconnected solar cell matrices. Each matrix includes a number of series-connected solar cells of the emitter-wrap-through type. This arrangement is intended to prevent the solar cells from being damaged, particularly in aerospace applications. It is proposed to produce each solar cell matrix from a single wafer on which a pn-junction and a number of front and rear contacts are formed before the wafer is placed on a masking glass and divided into a number of individual solar cells. Again, the disadvantage is the required masking glass.
The preceding WO 98/54763 specifies a solar cell in a semiconductor substrate where a certain contacting method is prescribed, but it does not propose dividing a larger semiconductor into a number of individual sections.
The object of the invention compared to the above discussed prior art is to provide a new solar cell arrangement for industrial application, particularly, but not solely, for obtaining a series-connected solar cell permitting a cost-effective production.
The independent claims specify how the aim of this invention can be achieved. Preferred embodiments are found in the sub-claims.
Therefore, the first fundamental principle of the invention is to provide a solar cell arrangement of series-connected solar sub-cells consisting of a semiconductor wafer that forms a common base material for all solar sub-cells and wherein a number of recesses is provided for delimiting the individual series-connected solar sub-cells where at least some of the recesses extend from the top surface of the semiconductor wafer to its bottom surface through the wafer itself and in that at most some bridge segments are left in continuation of the recesses as far as the wafer edge so as to mechanically interconnect the solar sub-cells.
Said bridge segments achieve that the respective solar sub-cells are adequately electrically separated but still having a mechanical cohesion that allows that the solar cell arrangement can be handled as a single piece of material. Using a few bridge segments allows a considerably better insulation of the sub-cells with a considerably better stability. xe2x80x9cQuasi-short circuitsxe2x80x9d occur only localized and are not distributed over the full length of the sub-cell. Also, with the bridge segments the use of an expensive filler material, such as polyimide, is easily eliminated and they can be configured so narrow that the electrical properties of the solar cell arrangement are minimally affected, if at all. Thus, it is proposed to use a crystalline semiconductor waver, preferably consisting of silicon, for producing all cells in the solar arrangement while the sub-cells are electrically insulated by means of partially thinning the wafer.
In accordance with the invention, partially thinning means that the semiconductor material is fully removed between two sub-cells so that at most some bridge segments remain and thus narrow, long holes or slits, respectively, are made in the wafer.
If on one side of the solar sub-cells one finger grid is provided each for the emitter and the base, it is preferable to arrange a bus bar on the finger grid for emitter and base such that two adjacent solar sub-cells are electrically connected via a common bus bar. Said bus bar may run over the bridge segments.
In the area of the bridge segments an insulating material, particularly an oxide coating, can be provided on the semiconductor material of the wafer, firmly bonded with the latter, and a strip conductor is provided on top of the insulating layer.
The solar cell arrangement can have solar sub-cells with at least two varying areas where particularly the sub-cells arranged on the edge differ in area from the sub-cells in the center. The result of such a configuration is an increase in the efficiency of the overall arrangement. This is because of an incomplete insulation of the sub-cells causing a physical asymmetry. Said asymmetry, in turn, causes the individual sub-cells to operate at different operating points, which means that unlike common modules, where the size of the individual cells should be as uniform as possible, a variation in the size of the individual sub-cells may increase the efficiency. It can be demonstrated that the cell located at the wafer edge and representing the negative pole of the solar cell arrangement should be decreased in area compared to the remaining sub-cells.
The selected area ratio depends on the quality of the insulation between the sub-cells and the selected geometry of the solar cell arrangement. The better the insulation the less the sub-cell areas should differ.
In a particularly preferred embodiment of the solar cell arrangement the bridge segments for the contact are designed to have an external component unit and/or forming a part of a contact element which is mounted laterally before the wafer is cut. This allows in the most consistent manner to completely sever the waver and to externally mount the bridge segments by means of a laterally mounted connection strip as an external component unit or by means of a carrier substrate.
It is also shown how the solar cell arrangement can be produced from series-connected solar sub-cells in that the recesses are produced by means of a waver cutter, with the use of a laser, by means of wire cutters, etching methods, sandblasting, water torches, ultrasound treatment, or a combination thereof.
The recesses can be produced particularly by means of a wafer cutter in that it is lowered stepwise to the wafer and moved across the latter, where the wafer cutter preferably is lowered to the wafer at a first site, moved across the wafer to where the bridge segment begins, moved across the waver at least once along the groove with a given lowering factor, and subsequently it is lowered further and moved again. This allows that the bridge segments are easily produced without a significant risk of breaking. It can also be provided for a wafer cutter to be lowered to the waver controlled and/or regulated preferably under rapid rotation, moving it through the depth of the wafer and then transversely across the wafer.
When the solar cell arrangement consisting of series-connected solar sub-cells is provided with a contact bank on the lateral wafer edge the bridge segments can be encompassed by said contact bank. The contact bank can be provided with metal bows that produce an electrical connection from one of the sub-cell rear contacts to the front contact of another sub-cell and thus permit a series-connection of the sub-cells and comprising recesses through which the separating tools can be guided.
Another option of obtaining the ability to handle and insulate series-connected solar sub-cells is to arrange a semiconductor wafer on a carrier substrate and subsequently providing it with continuous cuts for producing and insulating the sub-cells. The carrier substrate can be designed for externally connecting the solar-sub-cells and/or it can be provided with channels through which the separating tools can be guided for forming continuous recesses through the wafer without damaging the carrier substrate.